[CMake][Lit][unittests] Deprecate CMAKE_BUILD_TYPE in each build directory for unittests.

For example,
cur) unittests/ADT/Release/ADTTests
new) unittests/ADT/ADTTests

RUNTIME_BUILD_MODE can be substituted to CMAKE_CFG_INTDIR.

With Make and Ninja, the tree is not built with multiple configurations.
Then, including the build type in target directory doesn't make sense.
See also "How can I build multiple modes without switching?"
http://www.cmake.org/Wiki/CMake_FAQ
CMAKE_CFG_INTDIR is set to "."

With multiple-configuration-aware build system, like Visual Studio, each unittest is built on appropriate directory, for example,
unittests/ADT/Release/ADTTests.exe
CMAKE_CFG_INTDIR is set to build system's variable, like "$(Configuration)" or "$(OutDir)".

Thus, "--param build_config" is also deprecated.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173616 91177308-0d34-0410-b5e6-96231b3b80d8